(Sanskrit उपाधि) Literally, "addition." A limiting factor. A philosophical term used in a variety of ways, such as "substance, basis, matter, vehicle." In generally, upadhi refers to anything that conceals reality (the Absolute, Brahman).
A term used by Shankaracharya in the philosophy of Hindu Vedanta to point out the fundamental impermanence of the sheaths or bodies we use during manifestation, but which confuse our perception of what is real: the Innermost.
"The reflected image of the Sun expands when the surface of the water expands; it contracts when the water shrinks; it shakes when the water is agitated; it gets divided when the water is divided. It participates in all the qualities and conditions of the water, while the real sun is unchanging throughout. Even so, Brahman although never changing participates, as it were, in the attributes and conditions of the body and the other Upadhis or limiting adjuncts or vehicles within which He dwells, He grows with them, as it were, and so on, but not in reality." - Sivananda
"Sense-organs, physical body, mind, Pranas, intellect, etc., are the products of Avidya (nescience). They are Upadhis (limiting adjuncts). Negate them, sublimate or eliminate them through Vedantic doctrine Neti-Neti (not this, not this). What remains behind, the balance or the residue left is Atman or Brahman only." - Sivananda
"Matter is Eternal. It is the Upadhi (the physical basis) for the One infinite Universal Mind to build thereon its ideations." - Blavatsky, The Secret Doctrine
"This world of delusions appears good;
The fool surrounded by darkness, bound by 'Upadhi' [Substratum of being, or basis of individual existence],
Regards it as eternal.
To him who sees aright, it is all nothing." - Buddha, Udana 7:Cula
"[The Three Traitors] are the Demon of Desire, the Demon of the Mind, and the Demon of Evil Will. They are the three Upadhis, bases, or lunar foundations that are within each human being." - Samael Aun Weor, Magic of the Runes